Add to_dict method to model classes

[declaresub]

I want to save heron transactions to a database in json format. Thus it would be convenient if the various model classes had a to_dict or similarly-named method that returns a dict representation of a model object. Preferably the dict returned could be passed to Model.init to reconstitute the object.

[cesarferradas]

@declaresub thanks for the suggestion! You can always convert a python object into a dictionary by using vars(), so you could do that no problem. Because this is not always clear, I'm adding a to_dict() instance method for convenience in PR #11

[cesarferradas]

This method is now available in version 0.1.3: https://pypi.org/project/heron-data/0.1.3/

[declaresub]

Actually, you cannot always convert a python object into a dictionary using vars -- instances of a class created with a slots attribute, in particular. But the real problem with this implementation is that it doesn't fit the use case of serializing to JSON. A Transaction object has a merchant attribute of type Optional[Merchant]. This to_dict method will not convert the Merchant object to a dict.